It understood Kenya's security concerns, but it wanted to appeal to law-enforcement agencies to "uphold the rights of all those arrested and to treat them in a humane and non-discriminatory manner", UNHCR said.

Somalia's ambassador to Nairobi, Ali Americo, told the BBC Somali Service that he had discussed the arrests with Kenya's Interior Minister Joseph Ole Lenku at a meeting on Monday.

Mr Americo said he had been informed that most of the 2,000 people arrested were Somalis.

However, Ugandans, South Sudanese, Ethiopians and Eritreans were also among those being held, Mr Americo said.

The attackers targeted a busy bus stop and a food kiosk in Eastleigh last week

Police spokesman Masoud Mwinyi told the BBC he did not know how many people had been arrested.

However, the number was increasing as the security operation went on, he said.

Last week, Kenya's government ordered all Somali refugees living in towns to move into designated camps in a bid to end the attacks.

President Uhuru Kenyatta said the operation would continue to protect Kenya from further attacks by al-Shabab, which is linked to al-Qaeda.

"We are not fighting any religion or community. Our fight is against criminals who kill innocent children, women and men going about their everyday activities, including praying," he said on Saturday.

On 24 March six people died in an attack on a church near the port of Mombasa.

In September, at least 67 people were killed after al-Shabab militants took control of the Westgate shopping mall in the capital Nairobi for four days.